The present invention can be applied to an orthogonal frequency division multiplexing (OFDM) communication system scheme. A conventional OFDM communication system will hereinafter be described in detail.
FIG. 1 is a block diagram illustrating a transmission end for use in the conventional OFDM communication system.
According to the basic principles of the OFDM scheme, the OFDM scheme divides a high-rate data stream into many slow-rate data streams, and simultaneously transmits the slow-rate data streams via many carriers. Each of the carriers is called a sub-carrier.
The orthogonality exists between many carriers of the OFDM scheme. Accordingly, although frequency components of the carrier are overlapped with each other, the overlapped frequency components can be detected by a reception end.
For this purpose, data bits to be transmitted to a reception end are mapped to data symbols by a predetermined modulation module 101. A data stream composed of data symbols is converted to several low-rate data streams by a serial to parallel (SP) converter 102. The individual sub-carriers are multiplied by the parallel data streams by the sub-carrier mapping module 103.
Each of the data streams can be mapped to a time-area signal by the Inverse Discrete Fourier Transform (IDFT) module 104. The IDFT can be effectively implemented by the Inverse Fast Fourier Transform (IFFT) module. Thereafter, the data streams can be converted into a single data stream by the parallel to serial (P/S) converter 105. The CP (Cyclic Prefix) insertion module 106 inserts the cyclic prefix (CP) in the resultant data stream, so that the symbols can be protected and transmitted.
In the case of the above-mentioned OFDM communication system, a symbol duration of the low-rate subcarrier increases, such that temporal signal dispersion (i.e., relative signal dispersion in time) generated by the multi-path delay spreading decreases.
A guard interval longer than a channel delay dispersion interval is inserted between OFDM symbols, such that inter-symbol interference can be reduced. If a duplicate of some parts of the OFDM signal is arranged at the guard interval, the OFDM symbols can be cyclically extended, resulting in the protection of the symbols.
In the meantime, an Orthogonal Frequency Division Multiple Access (OFDMA) scheme for allocating resources to a plurality of users using the above-mentioned OFDM principles will hereinafter be described in detail.
The OFDMA scheme is indicative of a multiple access method for providing individual users with some parts of sub-carriers available for the OFDM modulation system, so that it can implement the multiple access. The OFDMA scheme provides the individual users with frequency resources, and the individual frequency resources are transmitted to several users independent of each other, so that they are not generally overlapped with each other.
The above-mentioned scheme for allocating resources to several users using the OFDMA communication system will hereinafter be described.
There are a variety of general resource allocation schemes, for example, a localized resource allocation scheme, a distributed resource allocation scheme, and a resource allocation scheme based on a resource block level.
The localized resource allocation scheme allocates a neighboring frequency band to a specific user. The distributed resource allocation scheme distributes resources to several users, allocates the resources to the individual users, and alternately allocates the resources to the individual users. The resource allocation scheme based on the resource block level is considered to be the combination of the localized resource allocation scheme and the distributed resource allocation scheme.
FIG. 2 is a conceptual diagram illustrating the localized resource allocation scheme.
Referring to FIG. 2, the localized resource allocation scheme allocates the resources to the individual users, so that the resources allocated to specific user are configured to be adjacent to each other and limited to a specific frequency band of the OFDM symbol. The above-mentioned localized resource allocation scheme uses sub-carriers of similar frequency bands, so that it can selectively use a variable modulation scheme or a coding scheme according to the channel condition. However, a frequency band allocated to a specific user is limited to a predetermined-range band, so that a diversity gain on a frequency axis becomes lower as compared to the distributed resource allocation scheme.
FIG. 3 is a conceptual diagram illustrating the distributed resource allocation scheme.
The distributed resource allocation scheme is indicative of a resource allocation scheme in which a specific sub-carrier of the OFDM symbol moves to other locations according to a predetermined hopping regulation. The frequency band allocated to a specific user ranges over a frequency area wider than that of the localized resource allocation scheme, so that a frequency-axis diversity gain can be acquired. The distributed resource allocation scheme has difficulty in applying the adaptive modulation and coding schemes, which are the most suitable for the selected channel situation, to the individual users.
FIG. 4 is a conceptual diagram illustrating the resource allocation scheme based on the resource block level.
The resource allocation scheme based on the resource block level is indicative of an intermediate format between the localized resource allocation scheme and the distributed resource allocation scheme. The resource allocation scheme based on the resource block level binds neighboring sub-carriers in the form of a single block, so that the localized resource allocation or the distributed resource allocation can be conducted in block units. Indeed, the resource allocation scheme based on the resource block level can locally allocate resources to the individual users, or can distributively allocate the resources to the individual users. Substantially, the user may have difficulty in discriminating between this localized resource allocation scheme based on the resource block level and the conventional localized resource allocation scheme.
In the meantime, in the case where the OFDM communication system transmits resources to the individual users and transmits the signal to the users, the reception end performs channel estimation. A general channel estimation method and a method for transmitting a reference signal used for the channel estimation will hereinafter be described,
The fading phenomenon occurs in a wireless communication system environment by a multi-path time delay. The above-mentioned channel estimation compensates for the signal distortion caused by an abrupt environment. change based on the fading phenomenon, and uses a transmission signal to the signal recovery. In order to perform the channel estimation, the wireless communication system conducts the channel estimation using the reference signal pre-recognized by a transceiver.
There are two kinds of usages of the above-mentioned reference signal under the OFDM-based wireless communication system, i.e., a first scheme in which the reference signal is allocated to all the sub-carriers of a predetermined period so that it can be transmitted as a preamble signal format to a destination, and a second scheme in which the reference signal is allocated between data sub-carriers.
In the case of using a signal (e.g., a preamble signal) composed of only the reference signal according to the first scheme, the reference signal has high density, so that the first scheme has a channel estimation performance higher that that of the second scheme.
However, the higher the reference-signal density, the lower the amount of transmission (Tx) data. In order to increase the amount of Tx data, the second scheme for allocating the reference signal between data sub-carriers is superior to the first scheme for transmitting the reference signal configured in the form of the preamble signal format. In the case of using the second scheme for allocating the reference signal between the data sub-carriers, the reference-signal density becomes lower, so that the channel estimation performance may be deteriorated. In order to solve this problem, there is needed an improved arrangement method capable of minimizing the degree of the channel estimation deterioration.
Therefore, a method for arranging the reference signal and transmitting the reference signal according to the arrangement method in the case of using the second scheme for allocating the reference signal between data sub-carriers will hereinafter be described.
If the reference signal is allocated between the data sub-carriers, the reference signal can be classified into three kinds of reference signals, i.e., a com-format reference signal, a block-format reference signal, and a lattice-format reference signal.
FIG. 5A shows the arrangement structure of the com-format reference signal for use in the OFDM system. FIG. 5B shows the arrangement structure of the block-format reference signal for use in the OFDM system. FIG. 5C shows the arrangement structure of the lattice-format reference signal for use in the OFDM system.
The above-mentioned com-format reference signal structure of FIG. 5A shows a method for transmitting the reference signal to only a specific sub-carrier every hour. This com-format reference signal structure transmits a reference signal every hour, performs interpolation of the reference signal in a frequency area every hour, and performs the channel estimation on the interpolated reference signal, so that it is inadequate for the frequency-selective channel. In other words, it is preferable that the above-mentioned com-format structure may be used for only a specific channel having a coherence frequency higher than Nfreq equal to a frequency-axis arrangement distance of the reference signal.
The above-mentioned block-format reference signal structure of FIG. 5B transmits the reference signal to all the sub-carriers during a specific period of time only. The block-format structure performs the interpolation in the time area, so that it encounters the interpolation error under a specific channel having a coherence time of more than Ntime equal to a time-axis arrangement distance of the reference signal.
The above-mentioned lattice-format reference signal format is indicative of an intermediate format between the com-format structure and the block-format structure, and arranges the reference signal in consideration of the coherence time, the coherence frequency, and the frequency efficiency based on the reference signal usage.
In other words, the lattice-format reference signal structure of FIG. 5C exemplarily shows a specific structure which uses a coherence frequency of more than Nfreq equal to the frequency-axis arrangement distance of the reference signal and a coherence time of more than Ntime equal to the time-axis arrangement distance of the reference signal. This lattice-format structure of FIG. 5C can minimize the number of reference signals, and performs the interpolation in time and frequency areas.
The above-mentioned lattice-format structure can reduce the number of reference signal whereas it is very sensitive to the interpolation method and the channel selective characteristics, so that it is preferred by the OFDMA scheme. Also, the lattice-format structure can easily change the arrangement method of the reference signal according to channel environments.
FIG. 6A shows an OFDM frame structure including a high-density reference signal. FIG. 6B shows another OFDM frame structure including a low-density reference signal.
In the case of transmitting the lattice-format reference signal, the low-density reference signal is transmitted to a destination as shown in FIG. 6A.
If an objective channel has strong frequency-selective characteristics and strong time-selective characteristics, the high-density reference signal can be transmitted to a destination as shown in FIG. 6B.
In the case of transmitting a plurality of reference signals as shown in FIG. 6B, the reference signals are closely arranged on a frequency axis under the channel having the high frequency-selective characteristic, and are also closely arranged on a time axis under the same channel, so that it is very resistant to the time-selective characteristic.
The reference-signal transmission structure of FIG. 6B reduces the time-axis arrangement distance and the frequency-axis arrangement distance of the reference signal by half as compared to the structure of FIG. 6A, so that it can also be used for another channel having the ½ coherence time Ntime and the ½ coherence frequency Nfreq. However, it should be noted that the amount of Tx data is reduced by the number of added reference signals.
The conventional OFDMA-based wireless communication system equally applies a reference signal having a predetermined pattern to all the frames, and transmits the reference signal, resulting in the implementation of channel estimation.
However, multiple users (i.e., a multi-user) have different channel environments, so that a fixed reference signal structure is unable to satisfy the request of the multi-user.
In the case of transmitting the high-density reference signal to increase the channel estimation performance as shown in FIG. 6B, an objective performance of a user who requests a high channel-estimation performance. However, it should be noted that some users may have wasted pilots.
In the case of transmitting the low-density reference signal to improve the system production yield as shown in FIG. 6A, there are many sub-channels, but an unexpected user having an increasing bit error may occur.
For example, a rapidly-moving user (i.e., a user having a high moving speed) experiences an abruptly-changing channel, and has a short coherence time. So, in order to acquire the channel estimation performance, the reference signal structure in which several reference signals are closely arranged is adequate for the above-mentioned rapidly-moving user.
However, some users may be in a halt status. If the user is in the halt status, the coherence time is long, so that he or she may acquire a superior channel estimation performance although the interval among the reference signals is long.
Therefore, the above-mentioned conventional wireless communication system for transmitting the reference signal using a single-pattern structure cannot satisfy a variety of user requests. In conclusion, a method for transmitting the reference signal having an appropriate reference-signal density in consideration of the reference-signal density and the channel estimation performance must be considered.